An ability to deliver therapeutically active agents to diseased tissues and cells while avoiding damage to healthy tissues and cells, or the identification of drugs that are pharmacologically selective for one tissue or cell type over another has presented a difficult and long-standing problem for physicians treating patients. This is especially true for cancer.
Cancer may be considered the result of rapid and endless division of diseased cells and the growth of cell clusters to form tumors. Malignant cells, spreading from a primary tumor mass and lodging elsewhere in the body to form a secondary tumor burden. Differences between cancer cells and healthy cells are subtle and historically most anticancer chemotherapeutic agents have sought to destroy tumor cells based on the rapid and extensive cell division rate characteristic or cancer.
Examples of cell division related targets are DNA intercalation or cutting agents, replication, transcription and expression and repair or polymerase enzyme activity inhibitors and microspindle polymerization poisons. Such agents include, but are not limited to, alkylating agents, antibiotics, antimetabolites, DNA intercalating agents, topoisomerase inhibitors, taxanes, vinca alkaloids, cytotoxins, hormones, podophyllotoxin derivatives, hydrazine derivatives, triazine derivatives, radioactive substances, retinoids and nucleoside analogs (specific therapeutic agents include, for example, paclitaxel, camptothecin, doxorubicin, vincristine, vinblastine, bleomycin, nitrogen mustards, cisplatinum, 5-fluorouracil and their analogues). However, healthy tissues such as bone marrow and the epithelial lining of the gut for example, also have rapidly dividing cell populations and chemotherapy agents typically fail to distinguish between these and other healthy and diseased cells. The result is dose limiting and even life threatening side effects that have become characteristic of cancer chemotherapy. For poorly aqueous soluble agents such as paclitaxel the use of emulsifying agents such as Cremaphor has been suggested. Cremaphor has been shown to further contribute to the adverse side effect profile of paclitaxel.
One approach to making chemotherapy more selective for cancer cells is the development of drugs that are based upon more recently discovered biochemical and metabolic differences between cancer and healthy cells. Such differences for example have now been described in receptor and signal transduction pathways and oncogenes and gene regulators that control growth and differentiation or regulate apoptosis. Other examples are tumor cell metabolic requirements for specific amino acids. Acute lymphoblastic leukemia cells for example are dependent on external sources of the amino acid asparagine. The enzyme asparaginase has been utilized to deplete circulating levels of asparagine in an attempt to treat disease. Newer classes of drugs, such as tyrosine kinase inhibitors are being explored and with promising results. Tyrosine kinase activity has been linked to receptors such as epidermal growth factor which may be upregulated in certain tumor types. Troublesome side effects and dose limiting toxicities as well as emerging drug resistance have persisted and remained problems even with these more selective agents.
Additional differences between cancer and healthy cells have also been observed in the expression of cell surface antigens. Monoclonal antibodies and their fragments have been extensively studied for the selective diagnosis and therapy of cancer either by direct binding of an antibody to its antigen or the delivery of radioisotopes or chemotherapeutic agents that have been conjugated to the antibody backbone. Typically monoclonal antibodies are specific for a limited number of cancer types and a “pancarcinoma” antibody has not yet been identified. Traditional cell division directed chemotherapeutic agents as well as newer signal transduction directed agents and monoclonal antibodies or their fragments may fail to penetrate fully into a tumor mass or accumulate sufficiently in tumor cells to achieve optimal results (i.e. the active agent is not sufficiently internalized in the tumor cells). Such failures are usually associated with the physical or chemical features of the agents including charge, size, solubility, hydrophilicity, hydrophobicity, and other factors. Cure rates remain relatively low for many solid tumor types and even modest improvements in life expectancy are considered significant.
It has been reported that accumulation of a chemotherapy agent into a tumor mass can be promoted by increases in the molecular mass of the chemotherapy agent. Lack of lymphatic drainage and other features of tumor associated vasculature such as leakiness are believed to play a role in this phenomena. Increases in molecular mass can be achieved by lipid acylation, conjugation to inert polymers such as polyethylene glycol, polyglutamic acid, dextran and the like or by encapsulation of drugs into liposomes or nanoparticles of various sizes and compositions. Particles below 1 micron in size are believed to pass through the leaky tumor vasculature and accumulate in the extracellular space of a tumor mass. Polymer conjugation or encapsulation can also be utilized to improve aqueous solubility or decrease plasma protein binding and accumulation into healthy tissue.
Liposome encapsulated drugs such as doxorubicin are currently in clinical use for treatment of AIDS related Kaposi's sarcoma and ovarian cancer. Polyethylene glycol or polyglutamic acid conjugated paclitaxel and polyethylene glycol conjugated camptothecin are presently in human clinical trials.
In general, liposome or nanoparticle encapsulation and polymer conjugation while enhancing drug accumulation in a tumor mass may actually slow or inhibit uptake or internalization of drug into tumor cells. Drugs are then left to diffuse out of degradable liposomes or nanoparticles. For polymer conjugation a prodrug strategy has been adopted. Decreased rates of plasma clearance of these formulations has also been reported and suggested to contribute to increase tumor mass accumulation. In a prodrug strategy active drug is released from a carrier as the conjugate circulates through the blood.
One effort at addressing the issue of selective tumor destruction is disclosed in U.S. Pat. No. 5,215,680. A moderately hydrophobic neutral amino acid polymer is labeled with a paramagnetic complex comprising a metal ion and organic chelating ligand. The labeled reagent is combined in solution with a surfactant mixture and then shaken in a gaseous atmosphere to form a gas in liquid emulsion or microbubble. Although principally employed for the enhancement of ultrasonic and MRI imaging, mention is made of pooling or concentrating the microbubbles in tumors to act as heat sinks as well as cavitation nuclei to disrupt the tumor structure.
Another approach has been to develop solid lipid nanoparticles as a delivery system for drugs, including for sustained release or oral delivery fromuations (See for example, Wolfgang Mehnert et al., “Solid Lipid Nanoparticles, Production Characterization and Applications”, Adv. Drug. Del. Reviews, Vol. 47, pp. 165-196 (2001) incorporated herein by reference. However, delivery systems employing solid lipid nanoparticles for tumor targeting have been problematical at least in part because of low drug-loading capacities, as well as unwanted accumulation in the liver and spleen or leaching of toxic agents remaining after particle formation.
Administering therapeutically active agents with an appropriate delivery vehicle that would limit accumulation in healthy tissues while promoting accumulation in a tumor mass and cellular internalization is highly desirable. With more efficient delivery, systemic and healthy tissue concentrations of cell division linked cytotoxic agents may be reduced while achieving the same or better therapeutic results with fewer or diminished side effects. Such delivery of agents with inherent degrees of tumor cell selectivity would offer additional advantages. Further, a delivery vehicle that would not be limited to a single tumor type but would allow for selective accumulation into a tumor mass and promotion of celluar internalization into diverse cancer cell types would be especially desirable and allow for safer more effective treatment of cancer. A delivery vehicle that would also allow for elevated loading capacity for the therapeutic agent would likely be a significant advance in the art.
Accordingly, there is a need for delivery vehicles which improve the efficiency of delivery of therapeutically active agents to targeted tissues including tumors with promotion of internalization into the targeted cells (e.g. cancer cells) preferably without the need for chemical modification or conjugation of drug and which have high therapeutic agent loading capacities. There is a further need for a method of preparing and using such vehicles for delivery of a wide variety of therapeutically active agents to targeted tissues and cells.